Auto-focus (AF) camera modules are commonly used in digital cameras and in mobile terminals having imaging capability. An automatic focusing algorithm is also commonly used in those camera modules for image capture. Many existing automatic focusing algorithms rely on hyperfocal distance focusing in image capture. In photography, depth-of-field (DOF) is defined as the distance in front of and behind the subject that appears to be in focus. Hyperfocal distance is such distance that if a camera is focused in that distance, then everything from half the focused distance to infinity will be within DOF.
Typically the DOF in a mobile imaging phone is very large, due to the small physical size of the camera module and the small lens aperture. In a typical mobile AF camera module, the hyperfocal distance is roughly 2 m (80 inches). Thus, when the camera is focused at 2 m, everything from 1 m to infinity appears to be in focus. For that reason, focusing at the hyperfocal distance can handle most image capture situations.
Nevertheless, AF cameras having an automatic focusing algorithm have a number of problems. These problems are also dependent upon whether the camera uses a two-stage capture button or a three-stage capture button. In a camera with a three-stage capture button, the first stage is the capture button not being pressed; the second stage is the capture button being half pressed for focusing purposes; and the third stage is the capture button being fully pressed for image capture. In a camera with a two-stage capture button, one stage is the capture button not being pressed and the other stage is the capture button pressed for image capture.
One of the problems associated with the automatic focusing algorithm is the long shutter lag in AF cameras. The user needs to wait for the automatic focusing algorithm to finish before the image can be captured. The moment that is intended to be captured can be lost because of the wait. The existing solution to this problem is that, if the capture button is fully pressed before the AF algorithm has finished searching optimal focus distance, then the image is captured with current focus distance, or focus is moved to hyperfocal distance and then the image is captured.
Another problem is that the camera shakes as the three-stage capture button is pressed fully to capture an image. The camera shake often results in a blurred image when pictures are taken indoor and in other low light situations where the exposure time needs to be sufficiently long. In a typical AF camera, one or more frames are captured after the capture button has been fully pressed, and the camera may be shaking during multiple frames after the capture button is pressed. Motion-blur most likely occurs in the image captured immediately after the capture button is fully pressed. The camera is shaken more easily because the third stage of button press requires a harder press than the second stage. As a solution to the problem, the user is instructed to try to keep the camera steady in low light situations, even when pressing the capture button.
Furthermore, in some imaging products, only two stage capture buttons are used. The problem is how to control AF and image capture with this kind of capture button. Typically, when the two-stage capture button is pressed, the automatic focusing algorithm is immediately initiated, but the image is not captured until the automatic focusing algorithm has found the focus. As such, the user has to wait for the automatic focusing algorithm to finish.
In capturing images of a moving object or capturing images in a low light situation, the existing image capture methods can be undesirable. Thus, it is advantageous to provide an image capture method that improves performance and quality with regard to the above-mentioned problems.